There are many unavoidable defects of prior amorphous silicon, e.g., low mobility, low stability, etc. Compared with the amorphous silicon, low temperature polysilicon has higher mobility and stability. Therefore, the low temperature polysilicon technology develops rapidly. However, the unevenness of the grain size and the irregular arrangement of the grains of the polysilicon will make the threshold voltage uneven, therefore influence the display quality of the display panel using the polysilicon film.